Military satellites have become indispensable tools in today's more challenging security environment. A major effort is underway to increase the timeliness and relevance of the photographic and signals intelligence that military satellites have to offer by making it available to field commanders and front-line troops. The large ground stations that are now required for satellite uplink and downlink therefore must evolve into rugged and mobile satellite receivers for soldiers and their vehicles.
Military satellites use time division multiplexing (TDM) to communicate. Satellite Digital Audio Radio Service (SDARS) systems also use TDM (see, Sirius Satellite Radio, “Satellite Digital Audio Radio Service (SDARS) System Architecture Document and Receiver Functional Specification,” Rev 1.3, May 01, 2002, incorporated herein by reference). Military satellites use differential quadrature phase shift keying (DQPSK). (DQPSK ensures that their transmitted signals are of constant power.) SDARS also uses DQPSK. Existing SDARS satellite receiver architectures and circuitry would therefore appear to be readily adaptable for use in a mobile military satellite receiver.
Unfortunately, this is not the case. As satellites orbit relative to their Earthbound receivers, the well-known Doppler shift causes their carrier frequencies to be offset. SDARS satellite receivers are designed to accommodate only a 78 kHz carrier offset. This is adequate for SDARS broadcasts, because SDARS satellites have high, generally circular orbits, meaning that the Doppler shift is relatively small. In contrast, some military satellites have low, highly elliptical orbits. This means that the carrier offset between the satellite and mobile receiver can be as much as 1.4 MHz. Conventional SDARS satellite receiver architectures and circuitry cannot accommodate this far greater offset.
Accordingly, what is needed in the art is an architecture appropriate for mobile military satellite receivers and a method of receiving military satellite transmissions. What is further needed in the art is a way to adapt existing SDARS satellite receiver architectures and circuitry to a military satellite communications environment.